An image forming apparatus using electrophotography may include a plurality of image carriers such as photoconductor, and a transfer member (e.g., transfer belt) facing the image carriers. The transfer member may travel in an endless manner in one direction.
In such an image forming apparatus, toner images having different color may be formed on each of the image carriers.
Such toner images may be superimposingly transferred onto the transfer member, and then transferred onto a recording medium (e.g., sheet), by which a full-color toner image may be formed on the recording medium.
In such a configuration, sometimes, toner images may not be correctly superimposed on the recording sheet by several factors. Such factors may include a deviation of light-path in an optical unit that scans the image carriers due to a temperature change, relative positional change of the image carriers due to an external force, for example.
If toner images may not be correctly superimposed on a recording medium when forming a fine/precise image by superimposing a plurality of color toner images, image dots having different color may not be correctly superimposed on the recording medium, by which a resultant image may have a blurred portion, which may not be acceptable as fine/precise image.
Furthermore, if such incorrect superimposing may occur when forming a character image on a non-white sheet, a white area may occur around the character image.
Furthermore, if such incorrect superimposing may occur when forming an image having a plurality of colored areas on a sheet, a white area may occur at a border of different colored areas or an unintended color image area may occur at a border of different colored areas.
Furthermore, if such incorrect superimposing may occur when forming an image having a plurality of colored areas on a sheet, unintended stripe images may occur on a sheet, and cause uneven concentration on an image, which is printed on the sheet.
Such phenomenon may unpreferably degrade an image quality to be formed on the recording medium.
Adjusting a writing timing of an optical unit of an image forming apparatus may reduce such drawbacks. Hereinafter such drawbacks may be referred to “superimposing-deviation of images” or “superimposing-deviation” as required.
An adjustment of writing timing of the optical unit may be conducted as below.
At first, a toner image may be formed on each of the image carriers (e.g., photoconductor) at a given timing, and then transferred onto to a surface of a transfer member such as transfer belt as detection image.
Such detection images may be used to detect an image-to-image positional deviation between toner images, to be formed on the transfer member.
A photosensor may sense the detection images and transmits a signal, corresponding to the detection image, to a controller of the image forming apparatus. The controller may judge a detection timing of the detection image based on the signal.
The controller may compute a relative image-to-image positional deviation value between each of the toner images based on the signal.
Based on a computed value by the controller, the controller may set an optical-writing starting timing for each of the image carriers (e.g., photoconductor) independently, by which a superimposing-deviation of images may be suppressed.
The above-mentioned image forming apparatus may employ a direct transfer method, which transfers toner images from image carriers to a recording medium, which may be transported by a transport belt.
In addition, the above-mentioned image forming apparatus may also employ an intermediate transfer method, which transfers toner images from image carriers to a transfer belt, and then to a recording medium. Even in such configuration, a superimposing-deviation of images may be reduced by adjusting a writing timing of an optical unit in a similar manner.
Toner images may not be correctly superimposed on the recording medium by the above-mentioned factors such as a deviation of light-path in an optical unit due to a temperature change, and relative positional changes of the image carriers due to an external force, for example.
In addition to such factors, other factors may cause an incorrect superimposing of toner images.
Other factors may include an eccentricity of image carrier, an eccentricity of drive-force transmitting member (e.g., gear) that rotates with image carrier, and an eccentricity of a coupling that is connected to image carrier, for example.
Specifically, if the image carrier or drive-force transmitting member may have an eccentricity, the image carrier may have two areas (e.g., first and second areas) on the surface of the image carrier with respect to a diameter direction of the image carrier.
For example, the first area of the image carrier may rotate with a relatively faster speed due to the eccentricity, and the second area of the image carrier may rotate with a relatively slower speed due to the eccentricity, wherein such first and second areas may be distanced each other with 180-degree with respect to a diameter direction of the image carrier, for example.
In such a case, first image dots formed on the first area of the image carrier may be transferred to a transfer member at a timing earlier than an optimal timing, and a second image dots formed on the second area of the image carrier may be transferred to the transfer member at a timing later than an optimal timing.
If such phenomenon may occur, first image dots formed on one image carrier may be superimposed on second image dots formed on another image carrier. Similarly, second image dots formed on one image carrier may be superimposed with first image dots formed on another image carrier.
Such phenomenon may cause incorrect superimposing of toner images having different colors.
In another image forming apparatus, a controller may conduct a speed-deviation checking and a phase adjustment control for toner images to reduce an incorrect superimposing of toner images.
The speed-deviation checking may be conducted by detecting a deviation of surface speed of an image carrier (e.g. photoconductor) when conducting an image forming operation.
The phase adjustment control may be conducted by adjusting a phase of each image carrier based on the speed-deviation checking.
In case of speed-deviation checking, a plurality of toner images may be formed with a given pitch each other on a surface of image carrier in a surface moving direction of the image carrier.
Such plurality of toner images may be then transferred to a transfer member (e.g., transfer belt) as speed-deviation checking image, and a photosensor may detect each of the toner images included in the speed-deviation checking image.
Based on a detection result by the photosensor, a pitch of toner images included in the speed-deviation checking image may be computed.
Bead on the computed pitch, a speed deviation per one revolution of each of image carriers may be determined.
Furthermore, another photosensor may detect a marking placed on a gear, which rotates the image carrier, to detect a timing that the image carrier comes to a given rotational angle.
With such process, the controller of the image forming apparatus may compute a difference between a first timing when the image carrier comes to the given rotational angle and a second timing when the surface speed of image carrier becomes a maximum or minimum speed.
Such process may be conducted for each of the image carriers.
After conducting such speed-deviation checking, a phase adjustment control may be conducted to adjust a phase of image carriers.
Specifically, a photosensor may detect a marking placed on a given position of a gear, which rotates the image carrier.
A plurality of photosensors may be used to detect a marking placed on a given position of gears, which rotates respective image carriers.
With such process, a timing when each of the image carriers becomes a given rotational angle may be detected.
Based on such information including rotational angle and speed-deviation of the respective image carriers, a plurality of drive motors, which respectively drives each of the image carriers, is driven by changing a driving time period temporarily to adjust a phase of image carriers.
With such phase adjustment of image carriers, image dots that may come to a transfer position at an earlier timing than an optimal timing, or image dots that may come to a transfer position at a later timing than an optimal timing, may come to a transfer position at an optimal timing.
With such controlling, a superimposing-deviation of images may be reduced.
Furthermore, if a pitch between adjacent image carriers may be set to a value, which is equal to a length obtained by multiplying a circumference length of image carrier with an integral number (e.g., one, two, three), each of the image carriers may rotate for an integral number (e.g., one, two, three) during a time when one toner image is transferred from one image carrier to a sheet at one transfer position and is moved to a next transfer position on a next image carrier.
Accordingly, by adjusting a phase difference of image carriers to substantially “zero” level, image dots may be preferably transferred to a transfer member at each transfer position.
On one hand, if a pitch between adjacent image carriers may not be set to a value, which is equal to a length obtained by multiplying a circumference length of image carrier with an integral number (e.g., one, two, three), each of the image carriers may not rotate for an integral number (e.g., one, two, three) during a time when one toner image is transferred from one image carrier to a sheet at one transfer position and is moved to a next transfer position on a next image carrier. In such a case, a different phase may be set for each of the image carriers respectively, by which image dots may be transferred to a transfer member from each of the image carriers at each transfer position defined by the transfer member and the each of the image carriers.
In view of such background, the inventors of this particular disclosure experimentally made a prototype image forming apparatus, which may conduct the above-explained adjustment control for writing timing of an optical unit, speed-deviation checking, and phase adjustment control. The inventors assumed that a superimposing-deviation of toner images may be effectively reduced by combining the above-mentioned controls.
However, such prototype apparatus showed a relatively greater superimposing-deviation of toner images in some experiments.
Such relatively greater superimposing-deviation of toner images may be caused as below.
A speed deviation per one revolution of an image carrier may be caused by an eccentricity of image carrier or drive-force transmitting member (e.g., gear), in general.
Therefore, when the image carrier or drive-force transmitting member may be replaced with a new one, a speed deviation per one revolution of image carrier or drive-force transmitting member may change.
Specifically, when a sensor detects a replacement of image carrier, a writing timing of an optical unit may be adjusted. Then, a phase of the each image carrier may be adjusted by a speed-deviation checking and phase adjustment control.
However, if such controls are conducted when the image carrier or drive-force transmitting member is replaced, a superimposing-deviation of images may become worse inversely.
Specifically, a writing timing of an optical unit, which may be adjusted to reduce a superimposing-deviation of images, may be determined based on a detection result of superimposing-deviation of images.
If one of the image carriers is replaced before adjusting a writing timing of an optical unit, a phase difference of image carriers may become unpreferable value due to such replacement.
Then, under the above-mentioned unpreferable condition of phase difference of image carriers, toner images may be formed on each of the image carriers.
Such toner images may be used for detecting a superimposing-deviation of toner images, and a writing timing of an optical unit may be adjusted based on the detected superimposing-deviation of toner images.
However, as above-mentioned, each of the image carriers may be in an unpreferable phase relationship with each other.
If a speed-deviation checking and phase adjustment control may be conducted after determining the writing timing of the optical unit under such unpreferable phase relationship for the image carriers, a following phenomenon may unpreferable occur.
Specifically, the writing timing of the optical unit, which is adjusted in earlier timing, may be unintentionally changed to unpreferable value by conducting the speed-deviation checking and phase adjustment control, by which superimposing-deviation of images may become worse.